Although manufacturers of galvanic cells for many years have attempted to produce a better cell having longer life, higher current drains and greater outputs by improving upon one or more elements of the cell, one area that has continued to be less than totally satisfactory has been the construction of the separator used in the galvanic cell.
In conventional alkaline type MnO.sub.2 dry cells, the cell construction generally consists of a metal container, suitably of steel, a mass or mix of MnO.sub.2 and graphite molded within the steel case, a separator adjacent to the MnO.sub.2 mass and an electrolyte and anode material in the center of the separator. The separator serves both as a barrier against migration of the depolarizer mix and the anode. In the past, it has been found convenient and practical to employ a cellulose based separator such as one constructed of paper, pulpboard, alpha cellulose, cellulose acetate, pasted kraft board, methyl celluose film and non-woven paper of cellulose fibers laminated to a similar mat of vinyl fibers. More recently, polyvinyl acetate sheeting has been employed as a separator material for its ability to prevent migration of depolarizer and to provide dimensional stability.
Conventionally, separators have been made by wrapping the separator around the sides of a bobbin and by folding the separator across the bottom of the bobbin before it is inserted within the cell. Usually, one or more washers were employed at the bottom of the bobbin to lock the folded edges of the separator against the bottom of the cell. However, separators of this construction suffer from many disadvantages. These separators, because they are wrapped around the bobbin before it is inserted within the cell, tend to loosen. As the separator must be tight to contain the particles of depolarizer mix, migration of these particles is likely to occur. Another disadvantage is that the folded bottom of the separator, even when washers are used to lock the folded edges are bulky and take up space within the cell.
To provide more room within the cell, the prior art developed a method of forming a separator by forcing a strip of separator material through a die with a punch and inserting the formed separator into the cell container which is mounted to the forming die. One such method is described in U.S. Pat. No. 3, 089,914 to Carmichael et al.
Although such prior art procedures provided reasonably satisfactory results with paper separators, it has been found that when the separators are constructed of relatively stiff, resilient materials, the separator walls, due to the resiliency of the material employed, tend to contract or coil inwardly toward the center of the cell thereby reducing the size of the opening into which the anode material is poured during the filling operation. This results in serious problems in high speed assembly of alkaline cells as the narrowed orifice reduces the free space available for anode material thereby providing a cell with a shortened life and frequently causes spill over of anode material onto the cathode with a resulting shorting and rejection of the cell.
It is therefore an object of the present invention to provide an improved separator construction for a galvanic cell.
Another object is to provide a method and apparatus for forming the separator in place within the shell of the galvanic cell.